1. Field of the Invention
The present invention relates to a scanning probe microscope. More specifically, the invention relates to a fully vibration-isolated probe microscope.
2. Related Background Art
A scanning probe microscope (SPM) such as a scanning tunneling microscope (STM) and a scanning atomic force microscope (SAFM) is for measuring a fine surface constitution in a local area on a sample with a resolving power of atomic level. There have recently been presented or proposed scanning probe microscopes incorporated with a conventional optical microscope to satisfy demands to observe a relatively large sample and to actually check a portion which is to be measured.
For example, a scanning tunneling microscope (as will be referred to as STM), which is disclosed in Japanese Patent Application Laid-open No. 2-163601 corresponding to U.S. Pat. No. 4,999,495, is provided with a support arm using a part of structure of a conventional optical microscope, to which an objective lens of the optical microscope and a detection portion of STM are individually attached. Another STM is disclosed in Japanese Patent Application Laid-open No. 3-4102, in which an objective lens and an STM detection portion are individually attached to an objective revolver, whereby alternate observations may be conducted by switching the revolver between an optical microscopic image and an STM image.
Although a very weak electric current is measured in the conventional STM's, an STM unit comprising a probe, a scanner, and an IV amplifier is set as exposed to an external environment or in a similar condition, while unprotected against external electromagnetic noises. Further, in spite that a sample is measured on an order of atomic level, external noises are readily transmitted to the STM unit, presenting a further problem.
In addition, a reduction in resonance frequency is caused by a cantilever structure of the support arm of an optical microscope or by mechanically operating portions such as a mechanism for vertical motion of the objective lens and a revolving part of the revolver. The reduction in resonance frequency leads to insufficient stiffness properties, which in turn results in easy transmission of vibrations of the microscope main body to the detection portion of the STM. The vibrations may cause an inability of achieving a resolving power of atomic level, presenting a still further problem.
It will be appreciated that an excellent vibration isolating property is necessary for a vibration isolating apparatus on which the STM is mounted. A surface plate of conventional vibration isolating apparatus usually has a square or rectangular mount plane, and is supported by four air springs or vibration isolating rubbers located on a square or rectangular frame. The air springs or vibration isolating rubbers have a natural oscillation frequency in a low frequency range, with that of the surface plate being in a high frequency range. In order to improve the vibration isolating property of vibration isolating apparatus, it is preferable to make a difference greater between the natural oscillation frequencies of air springs or vibration isolating rubbers and of the surface plate. Increasing the thickness of surface plate is a method to increase the natural oscillation frequency of the surface plate, but also increases its weight, presenting still another problem.